How do H/E and H3/E2 control coating system wear? - Insights gained from elevated temperature nanoindentation, scratch and impact tests

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Incipient plasticity in tungsten during nanoindentation: Dependence on surface roughness, probe radius and crystal orientation

The influence of crystallographic orientation, contact size and surface roughness effects on incipient plasticity in tungsten were investigated by nanoindentation with indenters with a range of end radius (150, 350, 720 and 2800 nm) in single crystal samples with the (100) and (111) orientations. Results for the single crystals were compared to those for a reference polycrystalline tungsten sample tested under the same conditions. Surface roughness measurements showed that the Ra surface roughness was around 2, 4, and 6 nm for the (100), (111) and polycrystalline samples respectively. A strong size effect was observed, with the stress for incipient plasticity increasing as the indenter radius decreased. The maximum shear stress approached the theoretical shear strength when W(100) was indented using the tip with the smallest radius. The higher roughness and greater dislocation density on the W(111) and polycrystalline samples contributed to yield occurring at lower stresses

Contact size effects on the friction and wear of amorphous carbon films

Since different properties of coating systems influence their friction and wear at different length scales contact size can play a critical role in microtribological experiments. In this study the behaviour of 3 different types of coating system which vary in terms of their thickness, substrate and mechanical properties has been investigated. The coatings were chosen for either their industrial relevance in automotive or MEMS applications, or as model coating systems. A wide range of nano/microtribological tests have been performed with different indenter geometries (tip sharpness), including single and repetitive scratch tests with unidirectional contact, and reciprocating wear tests, with depth and friction evolution monitored so that the relationships between failure mechanism and friction in coating systems with differing mechanical properties could be explored. The influence of surface topography on friction has been shown in ramped and constant load scratch tests. When fracture occurred resulting in a sudden increase in probe depth there was an abrupt decrease in friction which is ascribed to a contact area effect. In contrast, where deformation progressed through micro-wear a more gradual increase in depth can be associated with higher contact area and higher friction

Temperature dependence of indentation size effects, pile-up and strain rate sensitivity in polycrystalline tungsten from 25-950 C

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 ºC. Tests were carried out under high vacuum conditions as tungsten oxidizes in air at \u3e500 ºC. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were investigated at 25, 750, 800, 850, 900 and 950 ºC. Thermal drift assessed from the last 60% of a hold period at 90% unloading was typically ~0.05 nm/s and it did not vary significantly with load or temperature [1]. The hardness measurements were in good agreement with previous determinations by non-depth sensing hot microhardness. Please click Additional Files below to see the full abstract

Fretting wear of lubricated DLC coating systems

Diamond-like Carbon (DLC) coatings are well known for their use in protection against fretting wear due to their low friction and wear properties. DLCs are metastable, allowing them to graphitise under applied load to create a graphitic transfer layer which reduces friction. Their high intrinsic residual stresses also enable them to resist cracking effectively under fretting. Few studies have analysed the lubricated fretting performance of DLC coating systems. This work focuses on a series of lubricants with different additives and friction modifiers to explore their effects. This study analyses the performance of a DLC coating system (a-C:H) applied to hardened M2 tool steel and 316L stainless steel under loads of 20 and 40 N under dry fretting and lubricated fretting conditions. Lubricated uncoated substrates were also analysed for comparison. The counterfaces used were 10 mm diameter 52,100 steel balls. The lubricants tested included a base oil and a fully formulated oil, with and without the addition of MoDTC. Gross slip fretting was achieved using a bespoke electrodynamic shaker unit. Nanoindentation was employed to measure the mechanical properties of the coatings and substrates. Contact pressure and lubricant type had significant effects on the running-in behaviour of the coatings. Increased contact pressure led to instability in the running-in period. Lubrication reduced the dissipated energy in the contact, thereby decreasing wear. However, fully formulated oils and those containing MoDTC performed worse due to their higher viscosity, which impacted oil entrainment in the contact area. This study provides insights into the lubricated fretting performance of DLC coatings showing that these coatings can perform well, with the potential for further improvements with optimisations to the lubricant to the system. Performance improvements can be gained in automotive components such as high pressure bearings and gears

New instrumentation and analysis methodology for nano-impact testing

Nanoindentation testing has become increasingly popular for mechanical characterization of materials. This is motivated by the high versatility of the technique that allows testing of small volumes that could not be tested otherwise by macroscopic techniques, with minimal test preparation. The interest on nano-/microscale characterization of materials has been also extended to the study of high strain rate mechanical behaviour. One of the available techniques is nano-impact testing. It is carried out on a pendulum-based force-actuated, displacement-sensing device with the ability of performing energy-controlled impacts. The combination of conventional nanoindentation, for which a range of strain rates from 10-3 to 10-1 s-1 can be tested, with nano-impact provides a tool for materials characterization at the nano/microscale from 10-3 to 103 s-1. Regarding the analysis of nano-impact test results, there has been no consensus in literature over what material metrics to extract from the test. Several authors base the analysis of nano-impact test on the calculation of a dynamic hardness defined as change in kinetic energy throughout the impact divided by the residual volume of indentation [1-4]. However, there are two issues with the assumptions in which this equation is based. First, it only considers the change in kinetic energy and it neglects other important contributions like the work of impulse force. Then, it assumes that hardness is constant throughout the entire impact period. While for self-similar indenters this is true in the loading part, Cheng’s dimensional analysis shows that this is not the case in the unloading [5]. Therefore, the hardness calculated from this definition is not necessarily equal to the hardness under load commonly used in the instrumented indentation literature. To this end, an alternative analysis methodology is proposed. The analysis is based on the same definition of hardness under load commonly used in the instrumented indentation literature, computed as force divided by contact area. This way, the nano-impact hardness is directly comparable with results of conventional nanoindentation that use this definition. The instrumentation of the nanoindentation device with force-sensing capability was found crucial for the implementation of the analysis methodology. In addition, and in line with the nano-impact hardness definition in literature, an energy-based hardness is presented. The technique is assessed using finite element simulations and by testing six materials covering a wide range of mechanical behaviours. The FE simulations are used to assess the two energy-based definitions of hardness, the one in literature and the one proposed in this work. It was found that the literature definition leads to values that differ significantly from the ones obtained as force divided by contact area. On the other hand, the proposed energy-based definition provides values that match the ones obtained by force-approach. The experimental results are also in line with this conclusions. The literature energy-based hardness presents significant differences compared to the force-based hardness, which are higher for the more elastic materials. Furthermore, the force-based hardness computed from nano-impact results was compared with the hardness from conventional nanoindentation. A close match is found between both set of results. References [1] J.R. Trelewicz, C.A. Schuh, The Hall–Petch breakdown at high strain rates: Optimizing nanocrystalline grain size for impact applications, Appl. Phys. Lett. 93 (2008) 171916. [2] H. Somekawa, C.A. Schuh, High-strain-rate nanoindentation behavior of fine-grained magnesium alloys, Journal of Materials Research. 27 (2012) 1295–1302. [3] J.M. Wheeler, A.G. Gunner, Analysis of failure modes under nano-impact fatigue of coatings via high-speed sampling, Surface and Coatings Technology. 232 (2013) 264–268. [4] C. Zehnder, J.-N. Peltzer, J.S.K.-L. Gibson, S. Korte-Kerzel, High strain rate testing at the nano-scale: A proposed methodology for impact nanoindentation, Materials & Design. 151 (2018) 17–28. [5] Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Materials Science and Engineering: R: Reports. 44 (2004) 91–149. doi:10.1016/j.mser.2004.05.001

Development of high temperature nanoindentation methodology and its application in the nanoindentation of polycrystalline tungsten in vacuum to 950 ºC

The capability for high temperature nanoindentation measurements to 950 ºC in high vacuum has been demonstrated on polycrystalline tungsten, a material of great importance for nuclear fusion and spallation applications and as a potential high temperature nanomechanics reference sample. It was possible to produce measurements with minimal thermal drift (typically ~0.05 nm/s at 750-950 ºC) and no visible oxidative damage. The temperature dependence of the hardness, elastic modulus, plasticity index, creep, creep strain, and creep recovery were investigated over the temperature range, testing at 25, 750, 800, 850, 900 and 950 ºC. The nanoindentation hardness measurements were found to be consistent with previous determinations by hot microhardness. Above 800 ºC the hardness changes relatively little but more pronounced time-dependent deformation and plasticity were observed from 850 ºC. Plasticity index, indentation creep and creep recovery all increase with temperature. The importance of increased time-dependent deformation and pile-up on the accuracy of the elastic modulus measurements are discussed. Elastic modulus measurements determined from elastic analysis of the unloading curves at 750-800 ºC are close to literature bulk values (to within ~11%). The high temperature modulus measurements deviate more from bulk values determined taking account of the high temperature properties of the indenter material at the point (850 ºC) at which more significant time-dependent deformation is observed. This is thought to be due to the dual influence of increased time-dependency and pile-up that are not being accounted for in the elastic unloading analysis. Accounting for this time-dependency by applying a viscoelastic compliance correction developed by G. Feng and A.H.W. Ngan (J. Mater. Res. (2002) 17:660-668) greatly reduces the values of the elastic modulus, so they are agree to within 6% of literature values at 950 ºC

Micro-impact testing of AlTiN and TiAlCrN coatings

A novel micro-scale repetitive impact test has been developed to assess the fracture resistance of hard coatings under dynamic high strain rate loading. It is capable of significantly higher impact energies than in the nano-impact test. It retains the intrinsic depth-sensing capability of the nano-impact test enabling the progression of the damage process to be monitored throughout the test, combined with the opportunity to use indenters of less sharp geometry and still cause rapid coating failure. The micro-impact test has been used to study the resistance to impact fatigue of Al-rich PVD nitride coatings on cemented carbide. The impact fatigue mechanism has been investigated in nano- and micro-scale impact tests. Coating response was highly load-dependent. A Ti0.25Al0.65Cr0.1N coating with high H3/E2 performed best in the nano- and micro- impact tests although it was not the hardest coating studied. The role of mechanical properties, microstructure and thickness on impact behaviour and performance in cutting tests is discussed

Designing nanoindentation simulation studies by appropriate indenter choices: Case study on single crystal tungsten

Atomic simulations are widely used to study the mechanics of small contacts for many contact loading processes such as nanometric cutting, nanoindentation, polishing, grinding and nanoimpact. A common assumption in most such studies is the idealisation of the impacting material (indenter or tool) as a perfectly rigid body. In this study, we explore this idealisation and show that active chemical interactions between two contacting asperities lead to significant deviations of atomic scale contact mechanics from predictions by classical continuum mechanics. We performed a testbed study by simulating velocity-controlled, fixed displacement nanoindentation on single crystal tungsten using five types of indenter (i) a rigid diamond indenter (DI) with full interactions, (ii) a rigid indenter comprising of the atoms of the same material as that of the substrate i.e. tungsten atoms (TI), (iii) a rigid diamond indenter with pairwise attraction turned off, (iv) a deformable diamond indenter and (v) an imaginary, ideally smooth, spherical, rigid and purely repulsive indenter (RI). Corroborating the published experimental data, the simulation results provide a useful guideline for selecting the right kind of indenter for atomic scale simulations

Elevated temperature repetitive micro-scratch testing of AlCrN, TiAlN and AlTiN PVD coatings

In developing advanced wear-resistant coatings for tribologically extreme highly loaded applications such as high speed metal cutting a critical requirement is to investigate their behaviour at elevated temperature since the cutting process generates frictional heat which can raise the temperature in the cutting zone to 700–900 °C or more. High temperature micro-tribological tests provide severe tests for coatings that can simulate high contact pressure sliding/abrasive contacts at elevated temperature. In this study ramped load micro-scratch tests and repetitive micro-scratch tests were performed at 25 and 500 °C on commercial monolayer coatings (AlCrN, TiAlN and AlTiN) deposited on cemented carbide cutting tool inserts. AlCrN exhibited the highest critical load for film failure in front of the moving scratch probe at both temperatures but it was prone to an unloading failure behind the moving probe. Scanning electron microscopy showed significant chipping outside the scratch track which was more extensive for AlCrN at both room and elevated temperature. Chipping was more localised on TiAlN although this coating showed the lowest critical loads at both test temperatures. EDX analysis of scratch tracks after coating failure showed tribo-oxidation of the cemented carbide substrate. AlTiN showed improved scratch resistance at higher temperature. The von Mises, tensile and shear stresses acting on the coating and substrate sides of the interface were evaluated analytically to determine the main stresses acting on the interface. At 1 N there are high stresses near the coating-substrate interface. Repetitive scratch tests at this load can be considered as a sub-critical load micro-scale wear test which is more sensitive to adhesion differences than the ramped load scratch test. The analytical modelling showed that a dramatic improvement in the performance of AlTiN in the 1 N test at 500 °C could be explained by the stress distribution in contact resulting in a change in yield location due to the high temperature mechanical properties. The increase in critical load with temperature on AlTiN and AlCrN is primarily a result of the changing stress distribution in the highly loaded sliding contact rather than an improvement in adhesion strength